When high energy photons impinge on a detector, the inner shell electrons from atoms of the detector are ejected from the atom as “photoelectrons.” This phenomenon leaves the atom in an excited state with a vacancy (hole) in the inner electron shell. Outer shell electrons then fall into the created holes, thereby emitting photons with energy equal to the energy difference between the two states. Since each element has a unique set of energy levels, each element emits a pattern of X-rays characteristic of the element, termed “characteristic X-rays.” The intensity of the X-rays increases with the concentration of the corresponding element.
In many materials such as Cadmium Telluride (CdTe) or Cadmium Zinc Telluride (CZT) or the like, the characteristic X-rays primarily involve K-shell (closest shell to the nucleus of an atom) electrons. If the characteristic X-rays escape from the detector, the detector signal is incorrect and the loss of energy incurred manifests itself as errors in the output spectrum of the detectors. Thus, the measured spectral signal can be distorted and may cause artifacts in the reconstructed image.
The phenomenon of characteristic X-ray escape from the semiconductor detectors and the corresponding spectrum correction can be modeled using a Monte Carlo simulation. However, Monte Carlo simulations rely on random sampling in order to obtain numerical results in a heuristic manner, and thus tend to be computationally intensive. Accordingly, accurate image reconstruction can be achieved in real time by analytically modelling the characteristic X-ray escape phenomenon and correcting the measured output spectrum.